This invention relates to a code division multiple access (CDMA) receiver for use in a CDMA system and a method of controlling the CDMA receiver.
As a cellular mobile radio communication system, a wide variety of multiple access systems have been heretofore proposed and be adopted in the world. Among others, a recent tendency has been directed to a cellular mobile radio code division multiple access (CDMA) system which has a specific spreading code assigned to each channel and which will be simply called a CDMA system. In such a CDMA system, a modulated wave of an identical carrier frequency which is spread by each specific spreading code is transmitted as a radio signal from a transmitter side to a receiver side. Responsive to the radio signal, a CDMA receiver in the receiver side carries out synchronization operation by the use of each specific spreading code to identify a desired channel. In order to distinguish the channels from one another, different spreading codes are used to identify radio channels between the base station and the mobile stations.
In addition, it is to be noted that the radio signal is received through a plurality of paths, namely, a multipath in the CDMA system and, therefore, multipath fading should be removed from the radio signal in the CDMA receiver by accurately detecting a predetermined signal, such as a synchronization signal and/or a pilot signal.
Furthermore, it is to be considered in the cellular mobile radio communication system that each mobile station is moved through the radio service areas or cells from one to another with communication kept between each mobile station and a base station. In this event, the base stations must be switched from one to another without interrupting communication with each mobile station.
Taking the above into account, a CDMA receiver which is used in a mobile station has a rake receiver supplied with radio signals through the multipath and a searcher for searching such multipath signals to establish chip synchronization. In other words, the searcher is used to detect optimum reception timing from the radio signals and to inform the rake receiver of the optimum reception timing. This applies to the CDMA receiver which is used in each base station. Therefore, the following description will be mainly made about the CDMA receiver of the base station.
Practically, both the rake receiver and the searcher are given received data signals which are subjected to high frequency amplification and frequency conversion by a high frequency amplifier and an intermediate amplifier, respectively. In this event, the rake receiver is operable in response to the received data signals received through the multipath and comprises a plurality of finger receivers for demodulating the received data signal by the use of a spreading code into a demodulated signal. To this end, calculations are made about correlations between a spreading code of a desired channel and each received multipath signal to capture codes at each reception timing through each path. Thereafter, maximal-ratio combining is carried out to augment received signal strength. Such maximal-ratio combining is effective to alleviate an influence of the multipath fading and to improve a signal-to-noise (S/N) ratio.
On the other hand, the searcher has a plurality of correlators which are operable in response to the received data signals and delayed spreading codes to calculate correlation values between the received data signals and the delayed spreading codes and a plurality of adders for adding the respective correlation values to produce summed up correlation values. Moreover, the summed up correlation values are delivered to an effective path judgment circuit to determine an effective path and to supply the finger circuits of the rake receiver with reception timing signals representative of the effective path or optimum reception timing.
Herein, it is to be noted that the correlators of the searcher are in one-to-one correspondence to the adders.
Recently, recent attempts have been made to widen each of the cells in such a CDMA system to reduce a frequency of handoff operations for switching the base stations from one to another. This results in enlargement of a cell radius of each cell. Such enlargement of the cell radius brings about expanding or widening a searching range in the searcher of the CDMA receiver in each base station. In other words, the searcher must be widened in the searching range.
In order to widen or expand the searching range in the searcher, it is necessary to increase the correlators and the corresponding adders in number. Practically, the searching range is determined by a chip number of the spreading code and must have a predetermined resolution. In consequence, the correlators and the corresponding adders should be increased in number as the searching range is widened because the number of the correlators is equal to a product of the chip number and the resolution as well as the number of the adders.
Under the circumstances, the searcher is inevitably complicated in structure as the searching range is widened, because of a large number of the correlators and the adders. In addition, it is very difficult to make each base station small in size when the correlators and the adders are increased in number.
In other words, a conventional searcher has a fixed search window having a constant searching range where the searcher can search. The constant searching range is determined by the number of the correlators or the adders. The fixed search window is defined by a starting time instant which is equal to a reference reception timing of a base transceiver station (BTS) that has zero propagation delay. In other words, the fixed search window has no window offset. In order to expand or widen the constant searching range in the fixed search window, the searcher must increase the correlators and the adders in number.
It will be assumed that the CDMA receiver carries out reception operation under environment with a large propagation delay such as a case of having a large cell radius or the like. In this event, the conventional searcher must comprise a correlation calculator and an adder portion which have the large number of correlators and the large number of adders, respectively. This is because the searcher has the fixed search window and it is therefore necessary to make the correlators and the adders large in number in order to expand or widen the constant searching range. Accordingly, the conventional searcher is large in structure.
In addition, various prior arts related to the present invention are already known. By way of example, a method and an apparatus for performing search acquisition in a CDMA communications system is disclosed in U.S. Pat. No. 5,644,591 issued to Todd R. Sutton. According to Sutton, a large window of PN chip offset hypotheses are searched and if an energy signal is found that might indicate the presence of the pilot signal having one of the chip offsets of the large search window, then a search of a subset of offset hypotheses, or small window, is searched.
In a searcher according to Sutton, if the search window is swept and no hypothesis""s energy exceeds the detection threshold, then searcher controller would begin sweeping the next search window. In other words, the searcher according to Sutton mainly aims to search reception timimg of an effective path or the pilot signal. It will be assumed that the reception timing lies in one of the both ends of the search window by changing the reception timing of the effective path caused by moving a mobile station. In this event, it is feared that it cannot decide whether this path lies in a current search window or the next search window (or a previous search window). Under the circumstances, it is difficult to stably carry out reception processing.
Japanese Unexamined Patent Publication of Tokkai No. Hei 10-173,629 or JP-A 10-173629 discloses xe2x80x9cRECEIVING DEVICExe2x80x9d to precisely operate RAKE reception valid to code division multiplex system communication spread spectrum by setting a numerical value expecting an error range corresponding to a reception quality as a threshold value, and using it as the judgment reference of the output of a searcher circuit. According to JP-A 10-173629, a received signal is supplied to delays and despreading parts. The output of the delays are supplied to despreading parts. In the despreading parts, the despreading of the received signal is operated by the replica of a spreading code generated by a spreading code generating part, and secondary modulation is released. The despread signal is weighted according to a reception level by weighing circuits. Then, correlation values outputted by a sliding correlator and comparing parts are multiplied by the received signals, outputted by the corresponding despreading parts, and added by a synthesizing part. Then, the rate of a desired wave receiving power to an interrupting wave-receiving power to an interrupting wave-receiving power is calculated as quality information by an SIR measuring part.
However, JP-A 10-173629 may disclose a technical idea which uses the numerical value expecting the error range corresponding to the reception quality as the judgement reference of the output of the searcher circuit.
Japanese Unexamined Patent Publication of Tokkai No. Hei 10-173,630 or JP-A 10-173630 discloses xe2x80x9cCDMA CHIP-SYNCHRONIZING CIRCUITxe2x80x9d to attain synchronizing follow-up (tracking) on the fluctuation of a reception level, and to attain stable reception by selecting and demodulating a correlator output (a despread signal) in a timing with the most satisfactory reception quality in a constant time interval. According to JP-A 10-173630, a delaying means delays a despread code, so that it can be matched with a reception timing detected by a search means. Next, a shift register, having a plurality of output terminals, delays the delayed despread code in a constant delay interval. Then, a plurality of correlators operate the despreading of a received signal by calculating a correlation value between a digital base-band signal and each output of the shift register. Then, the outputs of the plurality of correlators are temporarily stored in a memory means. An optimal value detecting means and a selecting means read the memory in a constant interval, and selects the correlator output with the most satisfactory reception quality. Then, a synchronization detection means operates synchronization detection by using the selected correlator output.
However, JP-A 10-173630 neither discloses nor teaches concrete structure of the search means.
Japanese Unexamined Patent Publication of Tokkai No. Hei 10-200,508 or JP-A 10-200508 discloses xe2x80x9cTERMINAL EQUIPMENT FOR RADIO SYSTEM AND SEARCH METHODxe2x80x9d to shorten a search time when a demodulation output is not obtained in a CDMA system portable telephone system. According to JP-A 10-200508, a terminal equipment is provided with a next candidate memory that stores a code being a next candidate. When a demodulation output is not obtained and the code of the next candidate is stored in the next candidate memory, the code is partially searched based on the code to try demodulation. When the demodulation can be conducted, the demodulation is conducted by the code and only when the demodulation cannot be conducted, is the code searched with respect to all the phase by a searcher. Thus, the search time when no demodulation output is obtained is shortened.
JP-A 10-200508 may disclose a technical idea which carries out searching on the basis of the code of the next candidate stored in the next candidate memory when the demodulation output id not obtained.
Japanese Granted Patent Publication No. 2,853,705 or JP-B 2853705 discloses xe2x80x9cSPREAD SPECTRUM COMMUNICATION RECEIVERxe2x80x9d to eliminate erroneous detection of high level noises and to prevent deterioration of a characteristic when a plurality of paths in multipath phasing at low reception level and large fluctuation are detected in a spread spectrum communication receiver. According to JP-B 2853705, a search path detected in a search part and a tracking path tracked in a tracking part are held in a path acquisition/holding part, and a target signal is extracted by a front protection and a back protection. A correlation demodulation path selection part removes the path for signals other than the target one, selects the path to be demodulated and synthesizes rake in a rake part. A path front protection means sets a window of about one chip for the presence of a path and detects the existence of multi-paths when they continuously exist within the range of the window. A path back protection means detects the elimination of multi-paths when they do not continuously exist. The number of stages for executing front protection and back protection is suitably changed in accordance with an external situation.
JP-B 2853705 may disclose a technical idea for extracting a target signal by means of a front protection and a back protection in a path acquisition/holding part which compares a search path detected in a searcher part and a tracking path tracked in a tracking part.
It is an object of this invention to provide a receiver which is usable in a CDMA system, which is simple in structure and small in size.
It is another object of this invention to provide a receiver of the type described, which is capable of coping with widening a searching range of a searcher without increasing correlators and adders included in the searcher.
It is still another object of this invention to provide a receiver of the type described, which is usable in a base station and which is operable at a high speed.
It is another object of this invention to provide a searcher which is capable of searching optimum reception timing from a wide searching range without an increase of correlators and/or adders.
It is yet another object of this invention to provide a method of searching optimum reception timing from a wide searching range without an increase of correlators and adders.
According to an aspect of this invention, a method is for use in searching reception timing from received data signals which are received through a plurality of paths by a searcher in a CDMA system to detect the reception timing. The method comprises the steps of calculating, in a controllable search window having a constant searching range where the searcher can search, correlation values between the received data signals and spreading codes successively delayed, to produce summed up correlation values, of determining the reception timing from the summed up correlation values, and of shifting the controllable search window so that the reception timing lies in the controllable search window.
According to another aspect of this invention, a method is for use in receiving received data signals to produce a decoded data signal in a CDMA system and comprises the steps of calculating, in a controllable search window having a constant searching range, correlation values between the received data signals and spreading codes successively delayed, to produce summed up correlation values, of determining reception timing from the summed up correlation values, of shifting the controllable search window so that the reception timing lies in the controllable search window, of despreading the received data signals by using the reception timing to produce despread and detected data signals, of carrying out maximal-ratio combining of the despread and detected data signals to produce a combined data signal, and of decoding the combined data signal into the decoded data signal.
According to still another aspect of this invention, a searcher is for use in a CDMA receiver to search reception timing from received data signals which are received through a plurality of paths and comprises calculation means for calculating, in a controllable search window having a constant searching range where the searcher can search, correlation values between the received data signals and spreading codes successively delayed, to produce summed up correlation values, decision means for determining the reception timing from the summed up correlation values, and shifting means for shifting the controllable search window so that the reception timing lies in the controllable search window.
According to yet another aspect of this invention, a CDMA receiver is for use in a CDMA system to produce a decoded data signal in response to received data signals and comprises calculation means for calculating, in a controllable search window having a constant searching range, correlation values between the received data signals and spreading codes successively delayed, to produce summed up correlation values, determining means for determining reception timing from the summed up correlation values, shifting means for shifting the controllable search window so that the reception timing lies in the controllable search window, despreading means for despreading the received data signals by using the reception timing to produce despread and detected data signals, means for carrying out maximal-ratio combining of the despread and detected data signals to produce a combined data signal, and means for decoding the combined data signal into the decoded data signal.